This invention relates generally to infrared devices, and more particularly to a device for full duplex infrared communications.
Current Infrared Data Association (IRDA) products and other infrared products such as wireless headphones, which utilize scatter light from room surfaces, provide half duplex (one way) communications. Attempts to provide full duplex operation in general fail because of overload and intermodulation of the receiver while the transmitter is operating. This occurs even though the transmitter and receiver operate at different carrier frequencies (not different light wavelengths). When an object which reflects some light is nearby, such as a hand, very large amounts of transmitted light can be returned to the receiver.
It is desirable to consider a form of FM modulation on a carrier as a modulation scheme rather than a base band modulation scheme. While IRDA devices use base band modulation, the devices relate to line of sight applications with only a meter range. Here considerable optical gain can be utilized at the transmitter and receiver with only a thirty degree field of view requirement.
In normal noisy room environments, for instance, solid state ballasted florescent lighting, there is a huge light noise background. This is very intense at low frequencies since the ballast switching frequency is approximately 20 kHz which generates 40 kHz light noise with a high harmonic content. The intensity of noise is greatly decreased at approximately 300 or 400 kHz. In this frequency band, noise spikes occur every 20 kHz having a fine structure of two lines spaced about 2 kHz apart with finer noise lines on either side occurring every 120 Hz. In addition, each light fixture is not synchronized in frequency so that by the tenth harmonic, noise lines may be almost anywhere. With that type of ambient light noise background, the issue of photodiode shot noise and receiver front end noise become relatively less important for free space communications using scattered light. The transmitted light power has to be high enough so that at the receiver the desired signal and not the noise captures the FM limiter-discriminator. This in principle could be helped by using a spread spectrum approach, however spread spectrum circuitry uses up power which could be traded off for increased transmitted light power, which would accomplish a similar effect. Utilizing a much higher frequency to get above the noise harmonics is limited by the LED efficiency and the capacitive reactance of the photodiode. At higher frequencies the input stage noise level of the receiver and the photodiode shot noise due to ambient light intensity become increasingly more important.
Therefore, there is a need for an efficient full duplex infrared arrangement which minimizes transmitted light getting back into the receiver of the same device and reduces noise due to ambient light.
In accordance with the present invention, there is provided an optical device for full duplex free space light transmission. The optical device comprises a transmitting light source, having a transmitting optical axis and a radiated beam, and a receiving light detector having a receiving optical axis and a field of view. The transmitting optical axis and the receiving optical axis are nonparallel and substantially in the same plane.